For many years friction materials used in friction elements, such as disc brake pads, brake linings, clutch facings and the like for motor vehicles, have comprised mixtures of inorganic fibers, especially asbestos fibers, and particulate material which is largely inorganic, bonded together by an organic binder, particularly a phenolic resin. In some instances, organic fibers, such as cotton, and metallic strands, such as copper wire, have been included to impart certain desirable physical properties to the friction elements. Unfortunately, processes used in making friction elements from certain of such compositions involve undesirable manipulative steps which add unduely to production costs. Also, some of the compositions, prior to being formed into the finished friction elements, present hazards to health and to the environment.
More particularly, fabrication of disc brake pads and the like from friction compositions having a phenolic binder which has been rendered fluid by addition of an organic solvent requires the use of certain undesirable processing steps. The initial mixture of inorganic fibers, filler and phenolic binder has such fluidity as to evidence substantial flow unless confined in a container. Accordingly, an initial hot molding step involving the use of both heat and pressure, by means of which the mix is fabricated into uncured parts, e.g. disc brake pads, having sufficient rigidity to be stored prior to final cure, is required. Such initial hot molding step consumes energy in the form of heat and makes for longer than desirable production times represented by dwell time in the mold. The latter problem can only be overcome by purchase and installation of additional expensive processing equipment. In addition, the organic solvent must be recovered by a system which minimizes the possibility of physical harm to workers and atmospheric polution.
Although there are dry mixes from which friction elements may be fabricated by a simple cold molding operation followed by heating of the "green" preforms to cure the binder, such dry mixes, which usually contain relatively short asbestos fibers as well as fines, are dusty and thus are hazardous to the health of employees exposed thereto.
It is an object of this invention to provide a friction material from which friction elements, such as disc brake pads, clutch facings, and the like, having acceptable high temperature properties, can be fabricated.
It is another object of this invention to provide friction products which when subjected to high temperatures in use do not evidence high wear and fade characteristics.
Still another object of this invention is the provision of a novel friction material of low fluidity which can be formed into friction products by means of a simple cold-molding operation, followed by curing at elevated temperatures in a free standing condition.
Yet another object of this invention is to provide a novel friction material which in processing to obtain friction elements does not present the health and environment polution hazards evidenced by certain prior known friction materials containing phenolic binders.
These and other objects of this invention will become further evident from the following detailed description of preferred embodiments and the appended claims.